One way to increase engine efficiency is to reduce engine displacement and boost the engine. Lowering the engine displacement can reduce engine pumping work and friction, thereby increasing engine efficiency. Further, boosting can increase the engine output so that a smaller boosted engine can produce power that is similar to a larger displacement normally aspirated engine. In this way, engine efficiency can be increased while engine output is maintained.
However, downsizing and boosting an engine can also have drawbacks. For example, it may be more difficult to purge fuel vapors from a fuel vapor storage canister when an engine is boosted by a turbocharger or supercharger. In particular, during some operating conditions intake manifold pressure may be too high to create a desired amount of flow from a fuel vapor canister to the intake manifold. Further, during such conditions, the waste gate of a turbocharger may be at least partially open to limit boost so that the compressor speed does not exceed a threshold speed or so that engine exhaust back pressure is reduced to improve engine pumping efficiency. Thus, during such conditions the flow through the compressor may be relatively low. And, if the intake throttle is closed under such conditions to increase the flow of vapors from the fuel vapor storage canister to the intake system, engine power will be reduced. Thus, engine downsizing and boosting can make it difficult to purge fuel vapors during some conditions.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method for improving purging of fuel vapors from a fuel vapor canister. In one embodiment the present description provides a method for purging fuel vapors, comprising: decreasing a throttle opening area of an engine and increasing an amount of boost produced by a compressor coupled to the engine in response to an increased amount of fuel vapor, for example, an amount of stored fuel vapors exceeding a threshold amount, or a concentration of vapors in the purge flow exceeding a threshold level.
By reducing a throttle opening area and increasing an amount of boost produced by a compressor, vacuum produced via a venturi can be increased to improve flow of fuel vapors from a fuel vapor canister to an engine intake system while a desired level of engine output is maintained. For example, a venturi located in a compressor bypass loop can provide vacuum to draw fuel vapors from a fuel vapor canister to an intake system by using compressed air to create vacuum at the venturi. As boost pressure is increased, the pressure depression created at the venturi also increases, thereby increasing the potential for flow between a fuel vapor canister and the intake system. Further, reducing the throttle opening as the boost pressure increases allows the intake manifold pressure to be maintained at a substantially constant pressure so that the desire engine output can be maintained (e.g., so that the engine air charge is maintained even while the throttle and boost pressure are adjusted).
The present description may provide several advantages. For example, the approach may improve fuel vapor purging for a boosted engine. Further, the approach allows engine output to substantially match a desired engine output even when fuel vapors are being purged. In addition, the approach may limit engine energy consumption by only increasing the venturi efficiency when a stored amount of fuel vapors exceeds a threshold amount.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.